Oxygen sensors (O2 sensors) have heretofore been equipped with oxygen sensor elements, and the oxygen sensor elements have been accommodated in housing. In the case of bottomed cylindrical (cup-like) oxygen sensor elements, for example, an oxygen sensor element equipped with an oxygen-ion-conductive cup-like solid electrolyte, an internal electrode provided on the inner surface of the solid electrolyte, and an external electrode provided on the outer surface of the solid electrolyte are generally known.
While such oxygen sensor element brings an internal electrode into contact with the air such that the internal electrode serves as a reference electrode, it brings an external electrode into contact with a gas to be tested such that the external electrode serves as a measurement electrode. Thus, oxygen concentration in the exhaust gas from the internal combustion engine is measured.
In recent years, a variety of studies have been conducted on external electrodes in order to improve the performance of the oxygen sensor element. For example, an oxygen sensor element exhibiting improvement in the average particle diameter for crystalline materials of an external electrode and in the film thickness of the external electrode aimed at improvement in low-temperature operability and gas responsiveness has been disclosed (see, for example, Patent Document 1 or 2). According to such disclosure, a solid electrolyte constituting an oxygen sensor element is composed of stabilized zirconia, and a platinum (Pt) coating is provided on the solid electrolyte surface as an electrode material.
For example, an oxygen sensor comprising a solid electrolyte plated with a nonporous Pt cermet coating as a platinum coating used for an electrode has been proposed (see, for example, Patent Document 3). Another technique (i.e., an oxygen sensor comprising a solid electrolyte coated, on its surface, with a paste containing platinum particles via thermal treatment) has been proposed (see, for example, Patent Document 4). A further technique (i.e., an oxygen sensor prepared by allowing a platinum nucleus to deposit on a solid electrolyte surface, bringing a plating solution containing a platinum complex salt into contact with the nucleus to provide a plated coating, and heating the resultant to prepare a porous coating having open pores) has been proposed (see, for example, Patent Document 5).